Corrosion resistant treatment for hollow drill rods



CORROSION RESISTANTTREATMENI'EOR HOLLOW DRILL RODS' Bror .Ludvig Eugen .Rossander, Sandviken, :Sweden', as-

signor to Sandvikens Jerwerks Aktieblag,.Sandviken, sweden acorporation of Sweden t No Drawing. Application June 24, 1954 t x g Serial No, 439,159 'Claims priority, application Sweden June '29,. 1953 1 Claim. (Cl. 148-615) The present invention relates tohollow drill steels and especially to such steels, which arefexposed -to high fatigue stresses, as'for'ins tance percussion drill steels.

The use of hard metal cutting inserts ('sintered 'hard metal carbides) 'in the bits of ,drill steels has 'greatIyin creased the wear and life thereof and has madeit'possi' hie touse them in new and fasterdrilling ap'paratusfiand to increase the total bore depth per drill. The development of the drill steel itself, howeverhasnot kept pace with the improvement of the bit. In order further to increase the total bore depth perdrill it has become necessary to increase the life of the drill steel, which has hitherto, especially in drilling certain kindsof rocks, been relatively short due to fatigue ruptures caused by the greater stresses imposed by the drilling apparatus.

:It' has been shown, that wetting of the drill .steel, especially by means of a flushing liquid such asfor instancewater, considerablydecreases the fatigue strength in comparison with that of a drill 'steel used under dry conditions and the difference increases as the rate .of the percussions increases.

The reason for the decreased fatigue strength .of Wet or moist drill steel appearsto be corrosion and/or penetration of liquid into microscopic fissures because lateral ruptures appear to start atsuch points. 'These ruptures start mostly at the wall of the liquid passage but a substantial portion start also at the outer surface of the drill steel.

In order to reduce the mentioned tendency of the fatigue strength to deteriorate it has been proposed to coat the flushing channel with a corrosion resistant metal or with bitumen, paint, varnish or the like. The earlier methods, which have usually been relatively complicated and expensive, have not solved the problem because the improvement in fatigue strength has been found to be nil or negligible.

By means of the present invention, the above-mentioned difiiculties are avoided and a rock drill steel with a high fatigue strength in wet condition is obtained. Thus drill steels coated according to the invention and working with water as flushing medium have been found to have an increase of the life of 100% or more as compared to uncoated drill steels.

The purpose of the invention is to increase the fatigue strength of drill steel in wet condition to substantially the same level as its fatigue strength in dry condition.

According to the invention at least the flushing channel of the drill steel is coated first with a coating layer having absorbing qualities and firmly attached to the steel and then with a second coating layer having corrosion preventing properties. Preferably both the flushing channel and the other surface of the drill steel are coated in this manner.

Before the steel is provided with the above-mentioned protecting coatings it is cleaned, if necessary, for instance by sand blasting and/or pickling to free its surface from scale, rust, grease and the like. It is essential that the first coating layer shall be firmly attached to the steel,

for example by chemical bonding,and that it has such absorbing qualities, for example such a porosity that the second coating layer .ispartly absorbed by'the first coating layer. It has been found that the second coating layer when applied to such an absorbent and firmly attached first coating layer has no tendency to loosenor to be broken or cracked by the strong vibrationsltowhich drill steels are exposed.

An especially suitable protection of the steel is obtained by phosphate pre-treatment of the steel surface which gives a-compact grained, porous, chemically attached and strongly adherent first coating layer.

Other known coatings of similar type formed by chemical reaction have been found to give similar results, i. e.

strong adherence and sufficient porosity or texture for adsorbing and anchoring the second coating layer. Porous metallic coatings alsohave been found to be suitable asthe first coating'layer.

The corrosion preventing water-impervious second coating layer may consist of an oil orfat. A product having a high viscosity and a low water dissolvability should be used. It should form-a sufliciently strong'film that it will not be washed away by the flushing liquid. It has been found that'it is especially advantageous to use a wax for the second coating layer. Also coatings of.

rubber, 'both natural and synthetiqrubber-like materials, synthetic plastics, certain silicones, varnishes and paints have been found to provide useful corrosion preventing surface coatings.

It is advantageous insom coating layer a relatively thin or'low viscosity oil of the type which is attached to the first coating layer by 'electrostatic attraction and serves to repell the flushing liquid, for instance polar oils or oil solutions. A further improvement is obtained, if.the first coating layer is I treated witha corrosion inhibiting agentsuch as for example asolution of sodium orjpota s sium chromate or chromic acid. r In drilling certain kinds of rock the protecting coating on the outer surface of the drill Wears away relatively quickly. To meet this situation it is advantageous to provide the flushing channel of the rock 'drill steel with a protecting coating as describedabove, but to provide the outer surface of the steel with a coating of a steel having a greater resistance against corrosion and wear, for instance stainless steel or a chromized surface.

The drill steel usually consists of a hot: rolled lowalloy steel, for instance containing 0.1-l.5% C, 0.2- 1.4% Mn, 0.1-2.0% Cr, 0-3.5% Ni, 0.151.5% Si, and up to 0.5% M0, the remainder being iron. In its passage through the rolling mill furnace the work piece is exposed to the influence of the furnace atmosphere and its surface often becomes more or less decarburized. We have found that the fatigue strength of drill steel in wet condition is increased if the surface of the steel before the above-mentioned coating is applied is treated in such a manner, that increased hardness or compressive stresses are produced in the surface layers. Such compressive stresses or increases in hardness may be produced mechanically. For example, drill steels having decarburized surface layers may be cold drawn whereby the surface layer will be subjected to compressive stresses due to the contraction of the body of the drill steel when the drawing stress is relaxed. The provision of such a compressive stress in the surface layer depends on the fact that the limit of elasticity of the interior of the steel, the carbon content of which may be 0.8-1%, is considerably higher than the limit of elasticity of the decarburized surface layers. According to another mechanical method the surfacelayer is cold hammered, for instance by shot peening.

Compressive stresses in the surface layer may also be e instances touse as second introduced or-thehardness may be increased by means of chemical or physical-metallurgical methods, such as nitriding, case-hardening or surface hardening, for example flame hardening or hardening by inductive heating with a;substantially simultaneouscooling of the surface. 1

The following areillustrative examples of the invention.

1 Example 1 Hollow drill steel consisting of a hot rolled rod of a low-alloy steel with or without treatment as described above to create compressive stresses or increased hardness in the surface layer, is suitably cleaned by pickling or by sand blasting followed by pickling in 5-10% sulphuric acid. After washing and neutralization the steelis treated in a phosphate bath containing for instance phosphoric acid and zinc or manganese, i. e. the so-called Parker rust proofing process, whereby a thin, firmly attached, compact coating is produced on the steel. This treatment usually takes from a few minutes up to half an hour. Instead of the phosphate coating, the steel may be provided with a thin porous metal layer, for instance by spraying or contacting it with metal vapor. Before the second coating is applied the drill steel may be treated with a corrosion inhibiting solution, as for instance sodium or potassium chromate or chromic acid. After a dip in, for instance, 2% sodium chromate solution and drying, the second coating referred to above is put on, which preferably consists of a suitable wax or wax-like substance, such as parafifine wax, with waterrepellent qualities and with a tenacity and ductility chosen in order to obtain asatisfactory adhesion; with:

out any simultaneous tendency to .crack.

Also as noted above, other substances with similar qualities, such as certain plastics and oilor fat-products, may be used as the second coating.

Example 2 being iron, was normalized at 1040 C. to a hardness The steel was then nitrided in of about i 400 Brinell.

gaseous ammonia for about /2 hour at 530 C. The surface was then cleaned by pickling in 10%sulfuric acid at 70 C. and then pho-sphated by treatment in an acid phosphate solution for about 15 minutes (Parker rustproofing process). It was then treated in a 2% potassium bichromate solution and finally coated with wax having a melting point of about C.

The invention is usable also for other metal articles for which an increased fatigue strength in wet condition is desirable.

The scope of the invention is defined in'the appended claim.

I claim:

The method of improving the resistance to fatigue ruptures during drilling conditions .of a percussion'drill steel having a longitudinal bore therethrough for flushing water and consisting of a hot rolled low alloyed steel consisting of 01-15% C, 0.2-1.4% Mn, 0.1-2.0% Cr, up to 3.5% Ni, 0.151.5% Si, up to 0.5% Mo and the remainder iron with usual impurities, including in combin'ationthe steps of treating the drill steel to develop compressive stresses in at least the outer surface layer thereof, treating the resulting drill steel with a phosphate bath to form a'thin chemically attached phosphate coating having a porous crystalline structure on said outer surface and on the surface of said channel, treating said coating with a corrosion inhibiting solution and applying a wax-like coating to said phosphate coating.

References Cited in the file of this patent v UNITED STATES PATENTS 973,407 Connell Oct. 18, 1910 1,697,086 Pryce Jan. 1, 1929 2,181,361 Bean Nov. 28, 1939 2,310,239 Jernstedt Feb. 9, 1943 2,312,855 Thompson Mar. 2, 1943 2,418,935 Hutchinson Apr. 15, 1947 2,448,397 Schilling Aug. 31, 1948 2,476,345 Zavarella July 19, 1949 FOREIGN PATENTS Great Britain 'Oct. 15, 1952 

